CALGARY, Alberta  A nine-year study coinciding with burgeoning development in and around Banff National Park has found the area's grizzly bears have the lowest reproductive rate ever recorded in North America, scientists at the University of Calgary said.

The study of 71 bears between 1994 and 2002 also found that humans were responsible for more than 75 percent of female bear deaths and 86 percent of male deaths in the period.

"Basically, we (need) to have 19 out of 20 adult female bears in their reproductive years to survive into the next year," said Stephen Herrero, an environmental scientist at the University of Calgary and co-author of the study. "It's a tricky balance we'll have to maintain."

Female grizzlies do not reproduce until they are about 8 years old, and generally have litters of one or two cubs every four or five years. So the death of a single bear can have a significant impact on population trends.

The findings announced Thursday are to be published in the upcoming edition of Journal of Wildlife Management.

"This is definitely a wake-up call _ this says we have to be very careful with mortality because these bears are going to bounce back slowly, if at all," said Herrero.

The study was supported by business and environmental groups, as well as the federal and provincial governments. Herrero said it was important to keep everyone who "works and plays" in the parks focused on a common goal.

Practices adopted in the late 1980s, such as more fenced areas, eliminating human garbage and relocating bruins that have become accustomed to people are believed to have helped. Researchers believe at least one female grizzly a year has been saved by the measures.

A more extensive report is expected later this spring examining the grizzly populations in Alberta's less protected Eastern Slopes. That study, which is based on nine years of research plus monitoring in 2003 and 2004, will include policy recommendations.